Bodies are 60% (majority) water. Muscles, in particular, are composed of large quantities of water. So, if you are not staying hydrated, how do you expect your body to function in the gym how you want it to? It doesn't. It will lead to a poor session in the gym... and we don't want that, do we?
Water does 7 things in the body:
1) Transports - nutrients, oxygen, important hormonal messages and waste products (CO2).
2) Dissolves - water assists in dissolving protein and sugars. Water cannot dissolve fats (water and fat don't mix), but it encapsulates the lipids and transports them to their needed location in the body.
3) Cleans - water flushes out your important organs, most notably: kidneys.
4) Reacts - everything you do with you body is a series of chemical reactions. Water is a key component in a majority of these reactions.
5) Pads (Padding) - water pads your joints so the bones don't rub together. Ever notice how you feel a lot stiffer and your joints hurt a bit when you are dehydrated?
6) Lubricates - water lubricates organs and other internal areas of the body so they move freely within the body cavity.
7) Regulates your Temperature - ever heard of sweat? That's your body's way of cooling itself down. Your skin draws the water in your blood vessels to the surface of your skin where it will evaporate, cooling you down in the process.
If you're not hydrated, these functions slow down. If you go long enough, they shut down to the point of death (~3 days). That is the extreme, but getting dehydrated is not a fun time. There are also studies showing support for proper hydration assisting with cellulite and better skin overall.
But Andrew... how much do I need?
At the Iron Legion we usually suggest taking half of your body weight in ounces (oz) of water per day. Obviously this varies from person to person. A good rule of thumb is if your pee is pale yellow, you're good. Oh, enjoy those extra trips to the bathroom! :)